INTRODUCTION: In France March 14, 2020 a national lockdown was imposed in France for 55 days to prevent the spread of COVID-19 and all schools were closed. This study aimed to investigate the effects of home confinement as a result of  lockdown on the activity (physical activity and sedentary behaviors), and their determinants, on French children (6-10 years) and adolescents (11-17 years). METHODS: The National Observatory for Physical Activity and Sedentary behaviors launched an online survey from April 1, to May 6, 2020 using popular social networks and websites. It compared the level of physical activity (PA), sitting and screen time before and during the lockdown and identified the impact of the initial PA (active vs. inactive), sedentary (high vs. low) profiles of the participants and their housing conditions. RESULTS: 6,491 children were included in this study. Initially active children and adolescents decreased their PA more than those initially inactive (p>0.001), while those who met the sitting time recommendations increased more their sitting time during lockdown (p<0.001). The same applied to screen time (p<0.001). Living in an urban environment was associated with a decrease in PA (p<0.001), an increase in sitting time (p<0.001) and children's screen time (p=0.002) during lockdown. CONCLUSION: This study showed the deleterious effects of confinement caused by lockdown on physical activity and sedentary behaviors. Housing conditions were associated with lifestyle behaviors over this period of lockdown. Future public health policies should consider these results.